Hum reduction in alternating current receivers



June 6, 1933. BRADEN 1,912,755

HUM REDUCTION IN ALTERNATING CURRENT RECEIVERS Filed Feb. 19, 1930 2 Sheets-Sheet l -wwvw-- mow/r 6 v a I 1 --I INVEN'TOR N N RENE 'A. BRADEN ATTO-RNEY Patented June 6, 1933 UNITED STATES PATENT rules i F RENE A. BRADEN, 0F MERCHANTVILLE, NEW JERSEY, ASSIGNOR T0 RADIO CORPORA- TION OF AMERICA, A CORPORATION OF DELAWARE HUM REDUCTION IN ALTERNATING CURRENT RECEIVERS Application filed February 19, 1930. Serial No. 29,483.

My present invention relates to filament heating systems employing alternating current, and more particularly to a method of,

and means for, utilizing alternating current for heating the filaments of the space discharge tubes in a radio receiver and the complete elimination of the hum arising from such use.

it hasbeen realized, in the past, that the ideal radio receiver should be so designed as to permit the use of standard tubes of proved design with alternating current excitation of their filaments, the current being derived from home lighting circuits. When alternating current was so employed, however, it was found that a characteristic humming noise was produced in the receiver output. This noise was resolved into tone frequencies including a fundamental 60 cycle component and a double frequency component of 120 cycles. The cally accounted for by the observation that when the anode was connected to the filament at a point of potential variation, there was impressed upon the anode electrode a 60 cycle voltage variation; the latter being due to its connection to the filament circuit, and to the fact that the anode periodically combined with the positive leg of the filamenttoattract electronsliberatcd from the negative leg of the filament. ,The double frequency, on the other hand, was attrib uted to a phenomenon expressed as a bi'cyclic variation in initial emission velocity. That is to say, it was understood. that if the temperature of the cathode varied under the varying heat-producing electric current, the velocity of electron emergence would vary. It was also understood that this temperature effect, along With the effects of the voltage on the plate due to the latters connection with one side of the filament, occurred simultaneously.

To eliminate these hum notes various Heising in U. S. Patent 1,432,022, issued October 17, 1922 disclosed the connection from anode and grid to the'cathode at some point in r the filament circuit which has a potential equal to the average potential of the filament,

fundamental component was theoretimethods were resorted to. For example,

which in general was that of the middle point ofthe latter. Thus, the grid and plate circuits were connected to a resistance connected across the filament in such a manner that the average difference of potential at any instant between different portions of the cathode and grid, and the anode, due to the heating current, waszero. i

But these methods are only effective in eliminating or balancing out the fundamental, or 60 cycle, component of hum. The double frequency component still remains. And, especially, this true when space discharge tubes of high amplification factors are employed. Such tubes are of the screen grid type and of the pentode output type.

Now, I have discovered a novel method of eliminating the double frequency hum note, as Well as the fundamental component of hum, in receivers employing high amplification factor tubes, the filaments of which tubes are excited by alternating current. Briefly, the method involves connecting an impedance across the filament of a tube, connecting the grid and anode electrodes to the electrical center of the impedance, displacing the latter connection from the electrical center in the direction of the positive side of the filament while filament current is flowing, whereby there is introduced in the grid and plate circuits a voltage whose frequency is twice the filament supply frequency, and then utilizing this voltage to balance out the plate current fluctuation caused by the alternating filament supply.

There is, thus, provided a system applicable to audio amplifiers for purposes such as talking moving pictures, and public address systems, and also to radio frequency amplifiers to prevent 60 cycles and 120 cycles modulation of a signal, and in general to any circuit in which a filament type tube is operated by alternating current.

Accordingly, it is one of the main objects of my present invention to provide a method of, and means for, materially reducing hum noise in an electrical system employing a tube of a high amplification factor, and which tube has itsfilament heated by an alternating current,

which involves connecting a transformer to a a so source of alternating current and having the transformer secondary connected to the filament for heating it to incandescence, establishing an impedance path across the filament and the transformer secondary, connecting the anode and control electrodes of a .tube to the electrical center of the impedance path, providing an additional path of asymmetric properties in shunt with the impedance path and so arranged relative to the latter and the anode, control electrode connection to the imin such a manner that pedance path, that the electrical center is automatically displaced in the direction of the positive side of the filament, while filament current is fiowin g, while at the instance of zero current there is no displacement from the electrical center, whereby there is produced an efiect equivalent to the introduction in the control and anode circuits of a voltage whose frequency is twice the filament supply frequency, and then utilizing this equivalent voltage for balancing out the plate current fluctuation caused by the alternating current filament supply.

Another important object of the invention is to provide a method of operating an amplifier including a pentode output tube which has its filament excited by alternating current and which is normally productive of hum noise of a frequency double the exciting frequency which comprises feeding current from the supply source to the filament through a transformer and shunting the transformer secondary with a path including a path of adjustable impedances, connecting a second path of an asymmetric characteristic in shunt with the filament and the first path, connecting the control electrode and anode of the tube to both paths in such a manner that the control electrode and anode are connected to the electrical center of the combination only when no filament current is flowing, the electrical center being shifted in the direction of the positive side of the filament whenever filament current is flowing, and adjusting the first path in such a manner that the plate current fluctuation caused by the alternating current filament supply is balanced out and the double frequency hum materially reduced.

Still other objects of the invention are to improve the efiiciency of amplifiers embodying space discharge tubes having high amplification factors, and particularly to provide a method of heating filaments of such tubes hum is materially reduced, if not eliminated, and which is furthermore reliable in operation.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the

invention itself, however, as to both its organization and method of operation will best be understood by reference to the following de scription taken in connection with the drawings in which I have indicated diagrammatically one circuit organization whereby my invention may be carried into effect.

In the drawings,

Fig. 1 is a diagrammatic circuit of a pentode having its filament excited by alternating current,

Fig. 2 shows a circuit embodying the present invention,

Fig. 3 is a graphic representation illustrating' the operation of the circuit shown in Figs. 4 and 5 diagrammatically illustrate the theory of the operation of the invention as embodied in Fig. 2.

Referring to the accompanying drawings in which like characters of. reference indicate the same parts in the difierent figures, in Fig. 1 there is shown the conventional circuit for a pentode output tube for operation of the filament on alternating current. The pentode output tube is a type of tube possessing a high amplification factor and need not be described in any great detail, inasmuch as this type of tube is known to those skilled in the art. The filament of the tube is heated from an alternating current source through a transformer 2, the secondary of which transformer has connected in shunt with it a resistance 10, the latter being connected across the filament as well.

As shown in Fig. 1, input voltage is impressed between the control. electrode and a point in the filament circuit, and theoutput circuit or load 8 is connected between the anode and a point in the filament circuit. Anode current is supplied by a source of direct current B, a choke coil L being in series with the source. This connection is commonly used where it is not desirable to allow the steady component of the plate current to flow through the load circuit. The connection of the input'and output circuits to the filament is made through the contact 5 which is adapted to slide over the resistance 10.

It will be noted that the tube 1 includes a screen grid element 1" connected to the posi tive terminal of the direct current source B, while an additional electrode is provide for screening the anode 1, this additional electrode 1 preferably being connected to the center of the filament within the tube. This construction of the tube forms no part of the present invention except in so far as the various connections combine to produce the novel result to be pointed out hereinafter.

By adjustment of the contact 5 to the electrical center of the resistance 10, as explained heretofore, the fundamental 60 cycle component of hum can be balanced out. But, it is found that with a tube of the high amplification characteristics as the one described, a hum noise is still discerned, and this noise is due to a component which possesses a frequency double the exciting frequency; that is to say, 120 cycles. V

I will now describe an arrangement for materially reducing this double frequency hum, and at the same time balancing out the fundamental hum, it being understood that all the elements in Fig. 2 which correspond to the elements in Fig. 1 are designated by the same reference numerals. Instead of em ploying theresistance 10 with a movable contact 5,two separately adjustable rheostats 3 and 4 are connected across the filament and in shunt with the secondary of the transformer 2. The reason for using two adjustable rheostats in series instead of a fixed resistance with a sliding contactlas shown in Fig. 1) will be pointed out later. In addition, arectifier 6 is shunted across the rheostat 3, and a rectifier 7 across the rheostat 4, the control electrode and anode circuits of the tube 1 being connected to the mid-point 5;

Each of the rectifiers may comprise one or more pairs of elements of the well known copper oxide type. It is to be understood, however, that the rectifiers may be any other type of low resistance rectifier, well known to those skilled in the art. The rectifiers are connected back to back or face to face; that is t y. flow from the mid-point 5 through both rectifiers to the filament or in the opposite direction.

In order to explain the results obtained by means of the arrangement in Fig. 2, I have graphically shown in Fig. 3, by means of curves to now be described, the operation of the said arrangement. It will be noted that the curves are plotted on axes, the abscissa of which is Resistance in ohms across filament and the ordinate of which is Hum voltage across 5000 ohm load resistance.

Curve A shows the hum voltage across the load 8' in Fig. 1 with various values of resistance across the filament, no rectifier elements being employed. That is to say, this curve shows the results obtained with a conventional circuit such as shown in Fig. it is to be noted that the hum is substantially he same with different values or resistance 10 across the filament, the slider 5 being adjusted in each case to the point of minimum hum.

Curve B shows the hum voltage across the load 8 in Fig. 2, with various values of total resistance, that is to say, rheostat 3 plus rheostat 4, connected across the filament, the rectifiers 6 and? being employed, and each rectifier comprising one pair of copper oxide ele ments. Curve C shows the hum voltage for the same arrangements as were used for obtaining curve B, one of the rectifiers comprising one pair of elements, the other rectifier including two pairs of elements. Curve I) shows the hum voltage when each rectlfier consists of two pairs of elements.

In making the measurements for curves B, C and D,rheostats 3 and 4 were adpusted acin such a fashion that current can the other was adjusted to reduce the hum to i a minimum value. The hum voltage was then measured by a voltmeter connected across the load 8, which in the measurements was a resistance of 5000 ohms. There was, of course, no input voltage impressed on the input terminals, and these terminals were connected directly together.

The action taking place in the arrangement shown in Fig. 2 may be more readily explained by the use of figures illustrating the action of the circuit during different cycles of the filament heating current. Thus, in Fig. 4 the resistance 11 represents the rheostat 3 shunted by the rectifier 6, when the filament current flows in the direction indicated by the arrows, while the reference numeral 12 designates the resistance of rheostat 4 and rectifier 7 in parallel. It is to be noted that each rectifier presents a low resistance to a current flowing in one direction, and a high resistance to a. current flowing in the opposite direction. In other words, each rectifier may be considered as a device of asymmetric properties relative to the flow of alternating filament heating current.

It will therefore be seen, with the filament current direction assumed in Fig. 4, that rectifier 6 is a low resistance and rectifier 7 is a high resistance. Consequently, the equivalent resistance 11 is a low resistance, much smaller than the resistance of rheostat 3 in Fig. 2, and the equivalent resistance 12 1s onl slightly smaller than the resistance of the rheostat 4 in Fig. 2. When the filament current flows in the opposite direction, as shown in Fig. 5, the resistance of the rheostats 3 and 4 and rectifiers 6 and 7 may be represented by the equivalent high resistance 13 and low resistance 14.

From the aforegoing explanation it will be seen that the tape is displaced or shifted from the electrical center in the direction of the positive side of the filament, while lilainent current is flowing. On the other hand at the instance of zero current there is, of course, no displacement from the electrical center. Twice in each cycle, therefore, the control electrode and plate connection 5 is shifted from the electrical center toward the positive end of the filament and back to the said center. This produces an effect equivalent to the introduction in the control electrode and anode circuits of a voltage whose frequency is twice the filament supply frequency.

The magnitude of this equivalent voltage depends on the magnitudes of the resistances 3 and 4 in relation to the characteristics of the rectifiers. It will be obvious that by proper adjustment of the total resistance of rheostats 3 and 4,.this equivalent voltage can be made to balance out the plate currentfluctuation caused by the alternating current filament supply, with consequent material reduction of hum, if not elimination of the latter.

The sum of the resistances 3 and 4 must be adjusted to the right value to balance out the 120 cycle hum, while the ratio of the two resistances must be such that the 60 cycle hum is balanced. If the two rectifiers are identical, the resistances 3 and a will be approxi mately equal when the 60 cycle hum is balanced out. But if, for example, the rectifier 7 consists of two pairs of elements while 6 has only one pair of elements, it is found that the resistance 4 must be approximately twice as large as the resistance Thus, it is not necessary to have two e2:- actly identical rectifiers, for if one has higher resistance in the direction of current flow than the other, all that is necessary is to make suitable adjustments of the two rheostats 3 and a. In the above description, it has been stated that the point 5 shifts toward the positive end of the filament when filament current is flowing. However, if the two rectifiers are reversed in direction, the shift is toward the negative end of the filament. It is necessary to determine by tial which arrangement is correct in any given case.

In I-Ieisings circuit, the ($0 cycle hum is balanced out by connecting to the center point of a resistoracross the filament. In my circuit, the same effect occurs, that is, the 60 cycle hum is balanced out when the combination of resistors and rectifiers is properly adjusted. In addition, the 120 cycle hum is balanced out. The connection to the resistor across the filament does not necessarily go to the center however. The two resistances are roughly proportional to the resistances of the two rectifiers in the current-carrying direction. It is, therefore, not necessary to have two rectifiers which are exactly alike. This is illustrated by curve C in Fig. This current was made with a rectifier on one side, consisting of one pair of copper oxide plates and a. rectifier on the other side consisting of two pairs of plates. These plates were selected from a large number and were nearly alike. The results, therefore, are the same as would have been gotten ii two single rectifiers had been used, one having twice the resistance of the other.

One of the two rheostals is set at an arbitrarily selected value and the other is then adjusted to reduce the hum to a minimum value. lVhen this adjustment is made, the 60 cycle hum will be almost completely balanced out and the 120 cycle hum will probably be somewhat reduced. Then the first rheostat is adjusted to a new value and the second is again adjusted for minimum hum. The result then will be that the 60 cycle hum will be balanced out and the 120 cycle hum will be either reduced or increased. By continuing this adjusting, first one rheostat and then the other, an adjustment can be found which will reduce both hum and components to the smallest obtainable value.

The combination of a resistance and a rectifier, in parallel forms an impedance which varies between two definite values depending on the direction of the current. The percentage difference between the two impedance values depends on the magnitude of the resistance in relation to the characteristics of the rectifier. The percentage change can be calculated if the amount of current which flows through the rectifier in either direction with a definite applied Voltage is known. Then the proportional shift of the center tap can be calculated and hence the efiective voltage injected into the grid circuit. Thus, if the filament voltage is three volts and the center tap is shifted ten per cent, the effective peak voltage will be volt.

The shifting of the potential of the center connection may be toward either the positive or the negative-end of the filament, depending on how the rectifier-s are connected. In my experiments, I found that the potential had to be shifted toward the positive side. The opposite connection caused the hum to increase. However, there is no reason at present why some tubes might not require a shift toward the negative end of the filament. There is also a possibility that if the 120 cycle hum is out of phase with the supply voltage, reactors might be required in combination with the resistors and rectifiers.

It is to be understood that I do not limit this invention to pentode output tubes of any specific type of tube, it being within the scope of the invention to employ the method disclosed with any type of circuit employing tubes provided with filaments heated by alternating current.

lVhile I have indicated and described one arrangement for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is 1. An electric system comprising an electron discharge device having a thermionic cathode, an input circuit and an output circuit, a source ofalternating current connected to the cathode for supplying heating current thereto, a path of adjustable impedance in shunt to the source, a connection from said circuits to a predetermined electrical point on said path and a second path of variable impedance connected to said point for controlling the electrical value of said point.

2. An electron discharge device having a thermionic cathode and an input and output lot.

circuit, a-source of alternating current connected to said cathode for supplying heating current thereto, a conductive path in shunt to the source, a'connection from said circuits to a point on said path, and a secondpath of asymmetric characteristics connected to said point for electrically shifting said point in the direction of a predetermined electrical side ofthe cathode, while the heating current is flowing.

3. An electron discharge device having a thermionic cathode and an output circuit, a source of alternating current connected to said cathode for supplying heating current thereto a path of variable impedance in shunt to the source, and a connection from said output circuit to the electrical center, with respect to the cathode, of said path, and means having an asymmetric characteristic in shunt with said path for automatically shifting the electrical center in the di rection of the positive side of the cathode while heating current is flowing.

l. An electron discharge device having a thermionic cathode and an input circuit, a source connected to said cathodefor supplying alternating heating current thereto, a current-carrying path in shunt to the source, and a connection from said input circuit to the electrical center, with respect to the cathode, of the path, and asymmetric means connected to said path for displacing the electrical center of said path in the direction of a predetermined electrical side of the cathode whilethe heating current is flowing.

5. In combination, a space discharge tube including an anode, a cathode, and a control electrode, a source of alternating current for heating the cathode, an impedance shunted across said source and said cathode, a connection from a point on said impedance to the said control electrode and anode dividing the impedance into two portions, and a rectifier across each portion of said impedance.

6. In combination, a space discharge tube including an anode, a cathode and a control electrode, a source of alternating current for heating the cathode, a pair of variable impedances connected in series shunted across said source and sa1d cathode,

current, said means including a transformer connected to a source of alternating current and having the transformer secondary connected to the filament for heating it to incandescence, an impedance path acrossthe filament and the transformer secondary, means for connecting the anode and control eleca connectionv between said control electrode and anode and trodes of the tube to a point on the impedance path, an additional path of asymmetric properties in shunt with the impedancepath and so connected to the latter and the anode, control electrodeconnection to the impedance path, that the potential of the point of connection is automatically displaced-in the direction of the positive side of the filament, while filament current is flowing, while at the instance of zero current there is no displacement, whereby there is produced an effect equivalent to the introduction in the control and anode circuits of a voltage Whose frequency is twice the fila-- ment supply frequency, the equivalent voltage balancing out the plate current fluctuation caused by the alternating current filament supply.

8. An electron discharge device provided with a thermionic cathode, an anode, a control electrode, a screen grid and a shield grid at cathode potential, a source of pulsating current connected to the cathode for energizmg the same, an impedance path connected across the source and cathode, means connecting the anode and control electrode to a predetermined point on said path, and a second path of asymmetric characteristics connected across said cathode and source.

9. An electron discharge device provided with a thermionic cathode, an anode, a control electrode, a screen grid and a shield grid at cathode potential, a source of pulsating current connected to the cathode for energizlng the same, an impedance path connected across the source and cathode, means connecting theanode and control electrode to a predetermined point on said path, and a second path of asymmetric characteristics connected across said cathode and source, said point on said first path being connected to a point on the second path.

10. An electron discharge deviceprovidcd with a thermionic cathode, an anode, a control electrode, a screen grid and a shield grid at cathode potential, a source of pulsating current connected tot-he cathode for energizing the same, an adjustable impedance path connected across the source and cathode, means connecting the anode and control electrode to a predetermined point on said path, and a second path of asymmetric characteristics connected across said cathode and source.

11. An electron discharge device provided with-athermionic cathode, an anode, a control electrode, a screen. grid and a shield grid at cathode potential, a source'of pulsating current connected to the cathode for energizing the same, an impedance path connected across thesource and cathode, means connecting the anode and control electrode to a predetermined point on said path, and a second path of asymmetric characteristics connected across said cathode and source, said second path comprising at least two rectifiers connected in series.

12. An electron discharge device provided with a thermionic cathode, an anode, a control electrode, a screen grid and a shield grid at cathode potential, source of pulsating current connected to the cathode for energizing the same, an impedance path connected across the source and cathode, means con necting the anode and control electrode to a predetermined point on said path, and a second path of asymmetric characteristics connected across said cathode and source, said second path comprising at least two opposed rectifiers in series.

13. An electron discharge device provided with a thermionic cathode, an anode, a con trol electrode, a screen grid and a shield grid s at cathode potential, a source of pulsating 20' current connected to the cathode for energizing the same, an impedance path connected across the source and cathode, means connecting the anode and control electrode to a predetermined point on said path, and a second path of asymmetric characteristics connected across said cathode and source in shunt with said first path.

14. An electron discharge device provided with a thermionic cathode, an anode, a control electro'de,a screen grid and a shield grid at cathode potential, a source of pulsating current connected to the cathode for energizing the same, an impedance path comprising a pair of adjustable resistors in series connected across the source and cathode, means connecting the anode and control electrode to a predetermined point on said path, and a second path of asymmetric characteristics connected across said cathode and source.

15. An electrical circuit comprising a space discharge tube provided with an anode, electron emission element, control grid and screen grid, a source of pulsating energizing current for said elementconnected to the latter, an impedance connected across sald source and element, means for connecting said anode and control. grid to a predetermined electrical point on said impedance, and adevice of asymmetric character connected to said point and source.

16. An amplifier circuit comprising a tube provided with a control grid, anode, filament, and at least one auxiliary electrode, said tube having a high amplificationfactor, a source of alternating heating current, a transformer between the source and filament, a source of input energy connected to said filament and grid, a load circuit connected to said filament and anode, an impedance across the transformer and filament, a connection from a point on the impedance to said input source and load circuit, and a rectifier path in shunt with the impedance connected to said point.

17. A pentode output circuit comprising a tube having a thermionic cathode, anode,

grid, screen and a shield connected to the cathode within the tube, input and output circuits, a transformer connected to the oathode for supplying energizing current thereto, a path in shunt to the transformer secondary, a connection from the circuits to a point on the path having a potential substantially equal to average cathode potential, and an asymmetric conductor in shunt with said path and connected to said point.

18. A pentode output circuit comprising a tube having a thermionic cathode, anode, grid, screen and a shield connected to the cathode within the tube, input and output circuits, a transformer connected to the cathode for supplying energizing current thereto, a path in shunt to the transformer secondary, a connection from the circuits to a point on the path having a potential substantially equal to average cathode potential, and an asymmetric conductor in shunt With said path and a connection from the circuits to a point on the conductor, said last point being connected to said first point.

19. An amplifier circuit comprising a tube having a high amplification factor and including a cathode, input and output circuits, a transformer connected to the cathode for supplying alternating heating current thereto, a'pair of adjustable impedances in series connected in shunt with the transformer secondary, and a connection from said circuits to a point between said impedances, and a rectifier in shuntwith' each impedance.

20. In combination with an electron discharge tube including a cathode, control grid and an anode, said tube having a high amplification factor, a source of alternating current for energizing said cathode, a path of adjustable impedance connected in shunt with said source and said cathode for balancing out hum disturbance produced by the fundamental frequency of said source, means for connecting the control electrode and anode circuits of said tube to a predetermined point on said path, and a second path connected in shunt with said first path and being connected to said point, said second path having a variable electrical impedance for substantially eliminating hum disturbance due to a frequency double the said fundamental frequency.

RENE A. BRADEN. 

